The procedure of constructing phase diagrams of partially crystalline polymer-liquid systems, which is based on a difference in refraction indexes of a pure liquid and its solutions in partially crystalline and highelasticity polymer was proposed.It was shown in [1-4] that phase equilibria in partially crystalline polymer-liquid systems cannot be adequately refl ected by a diagram containing only one boundary curve describing dependence of melting points of polymer crystallites on the initial ratio of components of a binary mixture and taken (by analogy to the systems of two crystallizing low-molecular liquids) as a liquidus curve.Such diagram (Fig. 1) should contain at least one more boundary curve (line BD) refl ecting conversion of the initial two-phase polymer-liquid system into a singlephase system: a solution of a liquid in amorphous fi elds of a partially crystalline polymer. This is a phase line also in another sense. It is a right binodal branch refl ecting the extraction or osmotic equilibrium in a partially crystalline polymer-liquid system [4]. However, this is the fi rst sense, which is fundamentally important for understanding the essence of processes taking place in such system with a fi xed composition and variable temperature.Actually, let the binary mixture with a composition corresponding to point k is heated up to the temperature corresponding to point k 1 . In this case we shall meet two characteristics: the temperature Т Lp of the full dissolution of 0.4 weight fraction of liquid in amorphous fi elds of a polymer; the melting point Т mL of the last crystallite in the single-phase system of a polymer gel with grid nodes in the form of crystallites.Though line BD, which refl ects the dependence of Т Lp on the initial binary mixture composition, can be successfully constructed by results of a sorption experiment with excess liquid, it is methodologically important to have a possibility of its constructing by the direct determination of the temperature of conversion of a two-phase system into a single-phase system. The procedure described below allows us to do it. The procedure is based on the difference of refraction indexes of a swelling polymer and a liquid, which makes it possible to fi x the presence of this latter in the system by the presence of its meniscus.The difference in refraction indexes of coexisting phases decreases during the experiment fi rst of all due to the conversion of a pure liquid into a solution of polymer fractions in it, which do not participate in the formation of crystallites. Therefore it is expedient to extract preliminary these fractions from the polymer at a temperature close as much as possible to that corresponding to point B in the phase diagram (Fig. 1).Low-density polyethylene (LDPE) of 15083-020 grade (GOST 16377-77) with a melt index of 1.32 ± 0.2 g/10 min and a melting point of 111.2 ± 0.4°С used in this work was extracted by chemically-pure grade toluene at 64°С within 5 h.The installation (Fig. 2) consisted of magnetic stirrer 1 and thick-walled glass ...

Hydrostatic weighing of a polymer in an immersion liquid using low-density polyethylene as an example was used to show that a condition for the lack of recrystallization of partially crystalline polymers upon annealing is the attainment at this temperature of thermomechanical equilibrium that determines the ratio of the amounts of macromolecule elementary units existing in amorphous and crystalline regions during the crystallization of these polymers. A basis was found for the viewpoint that the Gibbs-Thomson equation is applicable only for estimating the dimensions of the last (not bonded to each other by connecting chains) crystallites disappearing at the true melting point.It is commonly thought in discussing crystallization processes of polymers with flexible chains from the melt, their additional crystallization (melting-recrystallization) during annealing, and complete melting [1-8] that these polymers are biphasic (crystalline and amorphous phases) and thermodynamically non-equilibrated (metastable).The experimentally observed manifestations of the metastability of partially crystalline polymers are: -a lower melting point (mp) than the equilibrium value ( 0 m T ) characteristic of ideal crystals; -a dependence of the mp on the heating rate and the thermal history of the sample; -additional spontaneous crystallization with a decrease of the glassification temperature of amorphous regions to the ambient temperature.Additional crystallization was clearly evident during successive stepwise heating of a partially crystalline polymer undergoing hydrostatic weighing in an immersion liquid. Additional crystallization of low-density polyethylene (LDPE) pressed at 160°C and crystallized in air at room temperature (mp = 111.2 ± 0.4°C) was distinctly observed in the range 58.5-110.2°C with such a heating regime [9]. Apparently only amorphization occurred in the range 29.3-58.5°C. Anyway, preliminary storage of the sample in a differential scanning calorimeter cell for 1 h at 40°C was accompanied by a decrease of its heat of fusion (ΔH m ) by 6% [glassification temperature (T g ) of this LDPE sample was about -53°C].Strictly speaking, these results were expected and agreed completely with existing concepts about the thermal behavior of partially crystalline polymers. However, the problem is that if the postulates of Gibbs regarding phase equilibria are strictly adhered to [10], then such polymers should be viewed as monophasic microheterogeneous metastable liquids in which the ratio between the amount of macromolecule elementary units located in the amorphous regions and crystallites is determined by the thermomechanical equilibrium conditions [11].Such treatment of the monophasic state of partially crystalline polymers differs in principle from that proposed earlier [3, p. 326] in that the amorphization process of such polymer liquids is considered to be a first-order phase transition complicated by a three-dimensional strain deformation relative to regions with long-range three-dimensional ordering in the space aro...

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The potential of hydrostatic weighing of partially crystalline polymers in an inert liquid during the study of their amorphization and crystallization processes over a broad temperature range was demonstrated using the low-density polyethylene-polymethylsiloxane system as an example.Professor Kirill Evgen'evich Perepelkin was, at a minimum, an extraordinary being among domestic (Soviet and Russian) polymer scientists studying problems of chemical fiber technology. The scope of his scientific interests was exceptionally broad. However, the relationships of the properties of natural and synthetic polymers to their structure and the formation conditions of the latter practically constantly interested him. Therefore, the two articles on this theme that are published below by representatives of the Ivanovo school of polymer chemists, some of whom had the pleasure of conversing personally with Kirill Evgen'evich, have become by a twist of fate a symbol of our parting with this remarkable person and scientist.Hydrostatic weighing [1, 2] is a method aimed at studying structure transformations in partially crystalline polymers that are due to the temperature and thermodynamically active liquids under the condition that the polymer remains in the solid phase during the course of the experiment and does not contain fractions that are soluble in the given liquid in the studied temperature range. The practical capabilities of the method were published earlier [1-3].Obviously replacing a liquid that is thermodynamically active relative to the polymer by a thermodynamically inactive one will automatically fulfill the insolubility condition regarding fractions of any molecular weight. Therefore, it becomes possible to measure the weight in the liquid of a polymer that exists also in the liquid phase, i.e., the melt.Such an experiment is interesting from two viewpoints. First, the true temperature of full amorphization (melting of the last crystals) of a partially crystalline polymer can be determined as the intersection of the temperature dependences of the solid and liquid (melt) polymers. Second, the nature and extent of structural transformations of the polymer matrix related to the change of its molecular packing density can be judged from the time dependences of the polymer density at a constant temperature.Herein the amorphization of low-density polyethylene (LDPE) in polymethylsiloxane PMS-20 medium is examined as an example of the practical implementation of these capabilities.The subject of the study was LDPE (15083-020 grade, GOST 16377-77) with melt index 1.32 ± 0.2 g/10 min (IIRT-5M instrument, 190°C, load mass 2.16 kg) as disks (20.0 ± 0.2 mm diameter, 3.0 ± 0.1 mm thickness) prepared by pressing granules at 160°C followed by cooling in air.

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